1. Field of the Invention
The present invention relates to a stainless steel sheet having a passive film good of solderability and electric conductivity.
2. Background Art
A stainless steel sheet represented by SUS 430 or SUS 340 has good corrosion resistance due to a passive film present on its surface. The passive film comprises oxides and hydroxides, and contains metallic components such as Si, Mn other than Cr.
Oxides and hydroxides in the passive film are thermally stable but unfavorable for low-temperature bonding such as soldering. In order to improve solderability of a stainless steel sheet, the passive film is dissolved by a fluxing agent containing a strong acid such as hydrofluoric acid, or the stainless steel sheet is precoated with a metal layer such as Cu for good solderability. However, such a corrosive fluxing agent causes contamination of a surface of the stainless steel around the soldered joint, and the soldered stainless steel sheet is necessarily washed to remove the contaminants. Formation of a metal layer of good solderability requires a plating step before soldering which causes an increase in manufacturing cost.
Oxides and hydroxides in the passive film are also electrically insulative. In this regard, the stainless steel sheet can not be used as such for a can body of a battery, a spring member for fixing a battery, a contact part for an electric circuit or an electromagnetic relay, and so on. Although a copper alloy has been used so far as material for an electric contact due to its excellent electric conductivity, it has insufficient corrosion resistance, and a contact part made of the copper alloy loses its electric conductivity due to generation of rust. In this regard, JP 63-145793A disclosed a stainless steel sheet coated with a Ni layer useful as a contact part. The proposed contact part has good of corrosion resistance derived from stainless steel, and defects caused by a passive film are eliminated by the Ni layer. However, formation of a Ni layer means increase in manufacturing steps and requires an expensive electroplating or electroless plating method. Due to the Ni-plating, there is a big burden on processing of liquid wastes. If the Ni-layer is formed on a surface of a stainless steel sheet with poor adhesiveness, it is peeled off during forming or handling the stainless steel sheet.